This invention relates generally to processes or methods for abating the emissions of hydrogen sulfide during the production of hydrogen sulfide-containing fluids from a subterranean reservoir and the subsequent processing of such fluids. The invention is particularly concerned with abating hydrogen sulfide emissions by converting hydrogen sulfide removed from the subterranean reservoir into an acid which is then used to control scale formation and/or to acidize production or injection wells. More particularly, the invention is concerned with controlling hydrogen sulfide emissions while inhibiting scale formation during the production of energy from a hydrogen sulfide-containing geothermal fluid, such as geothermal brine.
In many areas of the world where oil, gas and geothermal fluids are produced from underground reservoirs for subsequent use as energy sources, significant amounts of hydrogen sulfide, in some cases as much as 2.0 weight percent, are also removed with these fluids. Typically, hydrogen sulfide emissions must be abated in order to comply with environmental regulations. The methods used to achieve such abatement are quite frequently expensive and add significantly to the costs of producing and processing the oil, gas or geothermal fluids. This is especially true for processes in which geothermal fluids are removed from underground reservoirs and processed to generate electric power.
In general, processes by which geothermal brine is used to generate electric power have been known for some time. Geothermal brine from a producing well can be flashed to a reduced pressure to convert some of the water in the brine into steam. Steam produced in this manner is generally used in conventional steam turbine-type power generators to produce electricity. The remaining geothermal brine still contains significant energy, which can be captured by flashing the brine again for use in a steam turbine or by passing the brine through a closed-loop, binary fluid system in which a low-boiling point, secondary liquid (such as a hydrocarbon) is vaporized by the hot brine, and the resultant vapor used in a separate turbine-generator to produce electricity. Regardless of whether the brine is used for additional power generation, the geothermal brine is most commonly reinjected into the ground through a xe2x80x9creinjection wellxe2x80x9d for one or more reasons, such as replenishing the aquifer from which the brine was extracted and preventing ground subsidence.
Geothermal brines generally contain a high concentration of noncondensable gases, such as hydrogen sulfide, carbon dioxide, ammonia, and the like. In many localities such gases, particularly hydrogen sulfide, must be abated to comply with environmental restrictions. The use of hydrogen-sulfide abatement methods, such as the Selectox process, the LoCat process, and the methods described in U.S. Pat. Nos. 5,028,340 and 5,061,373, the disclosures of which patents are incorporated herein by reference in their entireties, can be quite expensive.
Geothermal brines also contain a high concentration of dissolved solid components, such as silica, metal sulfides and calcium carbonates. The solubility of most dissolved solid components in geothermal brine decreases with a decrease in brine temperature. Consequently, when a significant reduction in the brine temperature occurs or a loss of water due to a secondary flash takes place, supersaturation and precipitation of a portion of these components can result. Precipitates can deposit as a scale on any solid surface with which the brine comes into contact, such as a vessel, pipeline, or well in which the brine is confined. Scaling of the rock formation in the vicinity of the wellbore is also a well-documented occurrence.
High enthalpy brines, i.e., brines having an in-situ temperature above about 425xc2x0 F., typically have larger concentrations of dissolved solids than low enthalpy brines. The removal of larger amounts of heat and steam therefore produce significant levels of super-saturation and faster precipitation kinetics. These brines therefore tend to produce copious quantities of scale which can foul or plug conduits, heat-exchangers, vessels, injection wells, and/or the subterranean formation in the vicinity of the immediate reinjection wells.
It has been taught that strong acids, such as sulfuric acid and hydrochloric acid, can be added to highly saline brines in order to control scaling. Examples of such teachings can be found in U.S. Pat. Nos. 4,500,434 and 5,190,664, the disclosures of which are incorporated herein by reference in their entireties. Unfortunately, the use of such acids adds expense to the overall process of producing energy from geothermal fluids. Moreover, the transportation of the strong acids over public roads to the site of the geothermal power plant, which is frequently located in relatively remote regions of the world, increases the risk of accidental spill, injury and property damage.
Although, as discussed above, strong acids have been used to control scale formation, especially in processes where energy is extracted from geothermal fluids, and techniques currently exist for abating hydrogen sulfide emissions during the production of oil, gas and geothermal fluids, a need exists for more cost effective ways of abating hydrogen sulfide emissions and at the same time controlling scaling.
In accordance with the invention, it has now been found that the emissions of hydrogen sulfide during the production of oil, gas and/or geothermal fluids from subterranean reservoirs can be substantially avoided and/or significantly reduced by separating the hydrogen sulfide from produced fluids and converting the separated hydrogen sulfide into a hydrogen halide gas. The conversion is typically carried out by reacting the hydrogen sulfide with a halogen gas in a gas phase reactor or by contacting the hydrogen sulfide with halogen-containing solids, such as inorganic hypochlorites, chloroisocyanates, and bromochlorohydantoins. In order to make the hydrogen sulfide abatement process more cost effective, the hydrogen halide can be used, either directly or after it has been dissolved in water to form the corresponding halogen acid, to acidize the formation from which the oil, gas or geothermal fluids are withdrawn or to acidize production and reinjection wells in the same general vicinity. The hydrogen halide or halogen acid can also be used to control scaling in water produced from oil and gas reservoirs, or in processes for extracting energy from the produced geothermal fluids.
The on-site production of a hydrogen halide or a halogen acid from hydrogen sulfide removed from underground reservoirs with the oil, gas or geothermal fluids not only avoids the economic disadvantage of transporting considerable quantities of acid over great distances to remotely located facilities in order to utilize the acid for well acidizing or scale control, but also results in hydrogen sulfide abatement and thereby eliminates the cost ordinarily associated with conventional means of controlling such emissions. Moreover, the on-site production eliminates the potential environmental and safety hazards of transporting the acid over long distances on public roads in remote areas of the world.
A preferred embodiment of the invention involves the extraction of energy from a produced geothermal brine or fluid. In this embodiment, the geothermal fluid removed from an underground reservoir is flashed to produce hydrogen sulfide-containing steam and a geothermal brine containing dissolved solids. The steam is then used in a turbine generator to produce electricity. The hydrogen sulfide-containing steam exiting the turbine is condensed to produce a substantially moisture-free hydrogen sulfide which is then contacted with chlorine- or bromine-containing solids to produce hydrogen chloride or hydrogen bromide. These acid gases are then scrubbed to produce either hydrochloric acid or hydrobromic acid. The produced acid is added to the geothermal brine to reduce its pH, thereby preventing solids from precipitating to form scale during processing of the brine to remove its heat energy. The acid-treated brine also tends to remove previously formed scale. The site-produced acid, for example, can be added (1) directly into the production well to acidize the well and/or prevent scaling during removal of geothermal fluids from the reservoir, (2) upstream of the flash vessel to prevent scaling in the flash vessel during steam separation, or (3) in the piping upstream or downstream of the heat exchanger used to extract energy from the brine. In addition, the acid can be used to acidify reinjection wells or other producing wells in the same general vicinity in order to increase injectivity or geothermal fluid production, respectively. This integrated process results in the simultaneous abatement of hydrogen sulfide and inhibition of scale formation during the energy recovery process and results in a more cost effective overall operation.